III-V semiconductor quantum dot active material is of interest for creating optical devices such as lasers, optical amplifiers, switches, or spontaneous light emitters. Many applications exist in optical communications for these types of devices to generate, amplify, or switch high speed optical signals. Semiconductor light emitters are also of interest for other applications, such as a high power optical source for fiber amplifiers or solid state lasers, or for pattern reading such as in bar code scanners or compact disk data storage. Semiconductor devices are also of interest for solid-state lighting applications, for example, to replace incandescent or fluorescent lighting. All these applications could presumably be based on semiconductor devices that use quantum dots as the active material for generation or manipulation of light.
Fiber optic communication, in particular, relies heavily on semiconductor lasers to generate light signals, and amplifiers to increase the level of these signals. In lower cost applications it is desirable to have sources and amplifiers that operate without external cooling, and can therefore maintain stable operation over a temperature range that typically may span 0 to 85° C. Present commercial laser technology based on planar quantum well InP-based materials suffer from a strong temperature sensitivity in operating characteristics, which leads to difficulty in obtaining good performance over this temperature range.
In addition, laser sources that exhibit reduced lasing spectral linewidths are also of interest for fiber optic communications, as well as many other applications, because a reduced linewidth generally allows a longer fiber transmission distance, and reduced sensitivity of the laser operation to unwanted extraneous signals or internal operating characteristics. The semiconductor laser's linewidth is related to its alpha parameter, which also sets the laser's chirp characteristics (wavelength change) under direct modulation. Again, commercial InP-based planar quantum well lasers suffer from undesirable chirp characteristics that lead to a larger than desirable lasing linewidth, and a larger than desirable wavelength chirp under direct modulation.
Single wavelength lasers such as distributed feedback lasers, distributed Bragg reflector lasers, and vertical-cavity surface-emitting lasers are also important sources for 1.3 and 1.55 μm wavelength transmission down single mode fibers. There is a growing need for single wavelength lasers at these wavelengths that can operate uncooled. However, InP-based planar quantum well single wavelength laser technology also suffers from poor operation at the higher temperatures of about 85° C.
High power lasers also suffer in performance due to a greater than desired linewidth enhancement factor. For high power lasers the alpha parameter leads to changes in the lasing characteristics due to unwanted extraneous reflections and internal changes in the cavity characteristics that may occur at high drive levels. Therefore, it is also highly desirable to reduce the alpha parameter and temperature sensitivity in high power semiconductor lasers.
There has also been an unfilled need for semiconductor optical amplifiers with optical gain characteristics that are stable against temperature changes. However, InP-based planar quantum well optical amplifiers suffer from problems that are similar to lnP-based planar quantum well lasers. In addition, existing semiconductor optical amplifiers suffer from serious cross-talk problems when amplifying two signals at different wavelengths. This cross-talk problem, and limitations in the thermal characteristics of the planar quantum well active material, have limited the application of semiconductor optical amplifiers as compared to other amplifier schemes, such as erbium-doped fiber amplifiers.
There is also a need for an active material that confines electrons and holes to small volumes, for use in microcavity and photonic crystal devices. The performance of these types of devices is often dominated by edge effects that results from etch fabrication of the photonic crystal. Electron diffusion then usually limits the performance in prior art photonic crystal and microcavity devices (O. Painter, R. K. Lee, A. Yariv, A. Scherer, J. D. O'Brien, I. Kim and P. D. Dapkus, Science, June 1999).
Prior art semiconductor quantum dots are based on three-dimensional heterostructures, with each quantum dot able to confine charge carriers in a small volume with a size along each of the three dimensions that is less than the thermal de Broglie wavelength of the charge carrier. Because of this three-dimensional quantum confinement, each quantum dot heterostructure creates energy spectra for its charge carriers that are discrete levels due to its quantum confined charge carriers. In contrast, bulk or planar quantum well semiconductor materials contain quantum states for their charge carriers that form a continuum of energy levels over a significant energy range with respect to the semiconductors' thermal energy.
Early theoretical studies showed that quantum dots potentially have numerous advantages over more ordinary bulk or planar quantum well heterostructures for the application to semiconductor lasers (M. Asada et al., IEEE J. Quant. Electron. 22, 1915 1986; Y. Arakawa and A. Yariv, IEEE J. Quant. Electron. 22,1887, 1986; K. Vahala, IEEE J. Quant. Electron. 24, 523, 1988). These advantages are due to the optically active discrete energy levels of a quantum dot as contrasted to the continuous distribution of energy levels of bulk or planar quantum well active material. Specifically, quantum dot lasers are theoretically capable of higher modulation speeds, lower threshold current density, reduced temperature sensitivity, smaller spectral linewidths, and reduced wavelength chirp as compared to bulk or planar quantum well heterostructure lasers. In addition, quantum dots may reduce or eliminate the cross talk problem that exists in planar quantum semiconductor optical amplifiers, since quantum dots trap individual excitons and provide some isolation between optical gain at different wavelengths. The quantum dot attributes are potentially important for a wide range of applications based on light emitters, and are especially relevant to those limitations in present commercial lasers for fiber optics that use InP-based planar quantum well active material. Each of the potential attributes of quantum dot lasers results from modification of the continuous range of energy levels of bulk or planar quantum well active materials to the discrete distribution of energy levels for quantum dot active material.
Some researchers, however, have suggested that the quantum dot active material may have a fundamental limitation over bulk and planar quantum well lasers that also stems from its discrete energy levels, in that these discrete levels may cause a slow energy relaxation of electrons between the levels. The cause of the slow energy relaxation of electrons is suggested to occur because the quantum confined electron level energy separations can exceed the maximum phonon energy of the quantum dot and surrounding crystal. Nonconservation of energy in the relaxation of electrons between the discrete levels has then been proposed to lead to the so-called “phonon bottleneck.” This suggested phonon bottleneck could in principle eliminate the most important energy relaxation path of electrons from a surrounding heterostructure into the lowest energy quantum dot confined states, due to the limited emission of phonons. The belief that a phonon bottleneck may exist in semiconductor quantum dots has led some researchers to assert that this has also been a limitation in performance of lasers and other devices based on quantum dots (H. Benisty, et al., Phys. Rev. B 44, 10, 945, 1991; K. Mukai, et al., Appl. Phys. Lett. 68, 3013, 1996; D. Klotzkin et al., IEEE Phot. Tech. Lett. 10, 932, 1998; M. Grundmann, Appl. Phys. Lett. 77, 4625, 2000).
In 1994 Ledentsov et al. demonstrated a technique to realize InGaAs/GaAs quantum dot lasers based on self-organization of strained layer epitaxy. This technique is now widely practiced in many laboratories to create and study quantum dot lasers and other types of quantum dot photonic devices. In such a device a typically undoped quantum dot heterostructure formed through self-organization of strained layer epitaxy is sandwiched between p- and n-doped regions, from which it collects electrons and holes. Stintz et al. (Stintz et al., U.S. Patent Application 20020114367, Aug. 22, 2002) describe a quantum dot laser device in which an undoped quantum dot heterostructure is inserted into a p-i-n heterostructure, with the quantum dot heterostructure occupying the i (intrinsic) region as illustrated in FIGS. 1A and 1B. FIG. 1A shows a schematic of a p-i-n quantum dot heterostructure laser device, and FIG. 1B shows an expanded view of the devices quantum dot heterostructures active region. This strained layer growth technique to form quantum dot lasers also works in other materials, such as InP/InGaP, InAs/InGaAlAs, and InGaN/GaN.
Referring to FIG. 1A, metal layer 100 makes contact with insulator layer 110, which is provided, to direct current into the channel that forms the laser cavity. Layer 105 is an upper semiconductor cladding layer doped p-type, while layer 115 is a semiconductor quantum dot heterostructures active region. Layer 120 is a lower semiconductor cladding layer doped n-type, and layer 125 is a semiconductor substrate.
Next, referring to expanded view of the quantum dot active region illustrated in FIG. 1B, the active region is comprised of layers 130, 135, 140, and 145. Outermost layers 130 and 145 are semiconductor layers with energy gaps and refractive indices intermediate between the p- and n-type cladding layers 105 and 120. Sandwiched between layers 130 and 145 are layers 135 and 140, wherein layers 135 are semiconductor layers with an energy gap and refractive index intermediate between layers 130 and 145 and the quantum dot active material of layers 140.
The electrons captured from the respective n-region are desired to relax into the lowest energy electron levels of the quantum dots, and the holes captured from the p-region are desired to relax to the highest energy hole levels of the quantum dots, to form an optically active region of inverted level populations, and create optical gain. Charge neutrality in the p-i-n heterostructure requires that the injected electron number in the quantum dots be equal to the injected hole number so as to maintain charge balance. Numerous subsequent studies have shown that these types of p-i-n quantum dot lasers are capable of very low threshold current density, reduced optical chirp during modulation, and extended wavelength of operation within a given material system.
However, a serious drawback of p-i-n quantum dot lasers here-to-fore has been low optical gain and a strong sensitivity of their lasing threshold to temperature, in contradiction to early predictions. The low optical gain causes an increased sensitivity of lasing threshold current to temperature, as well as low power output. The temperature sensitivity of the threshold in semiconductor lasers can be described by the relationJth(T)=Jth(T)e(T−T′)/T0  (1)where Jth(T) is the threshold current density of the quantum dot laser, T and T′ are two temperatures of interest, and T0 is a parameter of the laser known as the characteristic temperature that characterizes the threshold current density change between T and T′. A low optical gain that is sensitive to temperature in turn causes a threshold sensitivity of the laser to temperature, which is an undesirable characteristic for uncooled applications. Prior studies have shown that quantum dot lasers suffer from a strong temperature sensitivity especially for temperatures above room temperature. Many studies have attempted to relate these drawbacks of low optical gain to a slow energy relaxation of electrons believed to be due to the phonon bottleneck.
Although prior works describe the quantum dot lasers based on a p-i-n heterostructures, there have also been two studies that investigate the influence of doping the quantum dot active material of Fabry-Perot semiconductor lasers to reduce absorption effects below threshold, and to attempt to overcome the believed phonon bottleneck. Yeh et al. (T N.-T. Yeh, et al. IEEE Phot. Tech. Leff., vol. 12, pp. 1123-1125, 2000) reported a study in which a small number of either donor (n-type) impurity or acceptor (p-type) impurity atoms are placed in InGaAs strain layered quantum dots. The purpose of their reported experiments was to study how decreasing the absorption of the quantum dots impacts the semiconductor laser performance. They found that placing impurity atoms within the quantum dots caused a modified crystal growth behavior. In measurements of the threshold characteristics of the temperature dependence of their lasers they found that either donor or acceptor impurities placed in the InGaAs quantum dots led to inferior temperature dependence of threshold as compared to undoped InGaAs quantum dot lasers of an otherwise similar heterostructures.